1. Field of the Invention
The present invention relates to a method for producing positive images by employing photosensitive microcapsules. More particularly, the present invention utilizes a low temperature exposure step to cause the microcapsules to be less susceptible to the reversal phenomenon caused by short time scale reciprocity failure.
2. Description of the Prior Art
Photosensitive imaging systems employing microencapsulated radiation sensitive compositions are the subject of commonly assigned U.S. Pat. Nos. 4,399,209 and 4,416,966 to The Mead Corporation as well as copending U.S. patent application Ser. No. 320,643 filed Jan. 18, 1982. These imaging systems are characterized in that an imaging sheet including a layer of microcapsules containing a photosensitive composition in the internal phase is image-wise exposed to actinic radiation. In the most typical embodiments, the photosensitive composition is a photopolymerizable composition including a polyethylenically unsaturated compound and a photoinitiator and is encapsulated with a color precursor. Image-wise exposure hardens the internal phase of the microcapsules as a result of the photoinitiator generating free radicals which initiate polymerization of the polyethylenically unsaturated compound by free radical addition polymerization. Following exposure, the imaging sheet is subjected to a uniform rupturing force by passing the sheet between a pair of pressure rollers.
U.S. Pat. No. 4,399,209 discloses a transfer system in which the imaging sheet is assembled with a developer sheet prior to being subjected to the rupturing force. Upon passing through the pressure rollers in contact with the developer sheet, the microcapsules rupture and image-wise release the internal phase whereupon the color-former migrates to the developer sheet where it reacts with a dry developer and forms a color image. The imaging system can be designed to reproduce monochromatic or polychromatic full color images.
U.S. Pat. No. 4,440,846 discloses a so called "self-contained" imaging system wherein both the image-forming agent and the developer material are located on the same substrate. In the system according to U.S. Pat. No. 4,440,846, the image-forming agent is encapsulated in a layer of pressure rupturable capsules, and the capsules are exposed and ruptured to cause the image-forming agent to contact and react with the developer to produce an image on the substrate.
A phenomenon known as short time scale reciprocity failure has limited the use of the aforementioned imaging systems to lower intensity radiation sources for longer periods of time. Certain high intensity light sources, such as lasers, useful in high speed copying have not been used effectively because the photohardenable compositions do not respond well to high intensity radiation.
At time scales less than 0.1 seconds, two types of reciprocity failure have been observed to occur with various types of photosensitive material. Failure of the first type occurs around 0.001 seconds for most systems and results in a progressive loss of sensitivity down to about 1 microsecond. The total loss at 1 microsecond, compared to 1 second, is expected to be about a factor of 10 for most systems. Whether further losses below the microsecond level occur by this type of failure is not known. The mechanistic cause of the failure is believed to be a chemical inefficiency due to high concentrations of free radicals. The radicals react with each other rather than doing the chemistry expected of them.
The second type of failure at short time scales is found in a few types of photosensitive materials. When this type of failure occurs, it is quite severe, causing an instant loss of a factor of 10 in sensitivity. This catastrophic loss has been observed to occur in different systems ranging from seconds to microseconds regions.
Most scientists who deal with photographic materials are familiar with the "H & D" (D log H) curve. The H & D curve characterizes the way in which a photographic material responds to light. An H & D curve is shown in the FIGURE. If the system is "well behaved," there is a relationship between energy or exposure and image density which defines the energy requirements of an exposure device. This region is represented by A on the FIGURE. In other areas, as is represented by B on the FIGURE, the imaging system is not well behaved and results in a broken or reverse proportionality between increased exposure and density. The region represented by B is the "reversal region" and depicts the second type of failure referred to above (i.e., short time scale reciprocity failure).
In well behaved regions of an H&D curve, for a given photosensitive material the degree of polymerization is essentially a function of exposure. Exposure may be quantitively expressed as a product of the intensity of the radiation source and the time of exposure. Accordingly, these two variables can be appropriately manipulated to provide a given exposure. In theory, as long as the desired exposure is obtained by any manipulation of intensity and time, a predetermined amount of polymerization should occur.
In regions where short time scale reciprocity failure occurs, the rate of polymerization slows to a point at which the photohardenable composition never fully polymerizes. A given level of exposure does not produce a corresponding degree of polymerization. When the photohardenable composition is microencapsulated and used in the aforementioned imaging systems, the system is unable to reach Dmin.
The reversal phenomenon varies widely amongst systems containing photosensitive materials. For some systems, reversal can occur only when subjecting the photosensitive composition to intensities and exposure times which cannot practically be achieved using present technology. For these systems short time scale reciprocity failure does not pose a serious problem. For other systems, short time scale reciprocity failure can pose a problem which must be overcome to enable the production of images. The problems are particularly apparent in systems which are subjected to high intensity radiation.
While not wishing to be bound by any particular theory, it is believed that short time scale reciprocity failure is in large part a function of the monomer used and the type and concentration of the photoinitiator. In systems where noticeable short time scale reciprocity failure occurs, it is hypothesized that termination reactions occur at a faster rate than the propagation reactions as a result of either the photoinitiator being depleted in the microcapsules before the monomer has had an opportunity to polymerize, or by the creation of a polymerization inhibitor or a radical scavenger in the microcapsules. Moreover, once the photosensitive material has encountered failure as a result of "reversal," the reversal effect cannot be corrected.
For some photosensitive materials, it has been difficult, if not impossible, to develop a system utilizing high intensity short time scale exposures because of the risk of reversal. When utilizing photopolymerizable systems as described above for copying documents, it is desirable to minimize the time of exposure to enable the production of copies at a fast rate. In systems which exhibit noticeable short time scale reciprocity failure, lower intensity radiation sources, and accordingly, longer exposure times are required to produce images. Accordingly, the number of copies produced per minute in such systems have been limited.
In addition, it has long been desired to utilize high intensity sources such as lasers as light sources for photosensitive imaging systems because of their monochromatic spectral characteristics and their ability to be easily controlled, such as by computer control as is known in the art. The use of lasers has been limited in some photosensitive systems as the high intensities of the lasers have caused some photosensitive materials to reverse as a result of short time scale failure.
Thus, there exists a need in the art for a method to reduce the susceptibility of a photo-sensitive composition to reversal upon exposure to thereby enable the production of a large number of copies per minute and the use of high intensity radiation sources.